1. Field of the Invention
The present invention relates generally to a microwave amplifier transmitting high-speed digital modulated waves, and more particularly to a microwave amplifier for use in a cellular phone, a base station, etc., utilizing wideband code division multiple access (WCDMA) or orthogonal frequency division multiplexing (OFDM).
2. Description of the Related Art
Reference is now made to FIG. 8 which illustrates a circuit of a conventional microwave amplifier. The microwave amplifier is designated generally at 10 and comprises a transistor Qd in the form of, e.g., a bipolar junction transistor (BJT); an input matching circuit 11 connected to a base of the transistor Qd; and an output matching circuit 12 connected to a collector of the transistor Qd.
A microwave signal output from a signal source 13 passes through an internal impedance R1 of the signal source 13 and is input to an input terminal In of a microwave amplifier 10. The microwave signal input to the input terminal In is fed via the input matching circuit 11 to the base of the transistor Qd, with the resultant amplified microwave signal being fed from the collector of the transistor Qd though the output matching circuit 12 to a load R2 connecting with an output terminal Out.
The bipolar junction transistor for use in the microwave band has an input impedance including a resistance component and a reactance component. On the contrary, the internal impedance R1 (if a transmission line intervenes, its characteristic impedance) of the signal source 13 normally includes only the resistance component of 50 ohms. Hence, to ensure an effective feed of microwave signals from the signal source to the transistor Qd, the input matching circuit 11 effects an impedance matching.
The input matching circuit 11 of FIG. 8 is formed of a two-stage xcfx80 circuit consisting of inductors L51, L52 and Lb and of capacitors C51 and C52. On the other hand, the impedance matching between the output impedance of the transistor Qd and the load R2 is effected by the output matching circuit 12 having the same configuration.
The input matching circuit 11 includes an inductor Lb whose one end is connected to a bias power source Vb2 to feed a bias current to the base of the transistor Qd. In this case, to raise the power efficiency of the microwave amplifier 10, it is desirable to cause the transistor Qd to effect a class B operation by which little or substantially no bias current flows across the base and emitter of the transistor Qd. Due to the output distortion which may increase in the class B operation, however, the transistor Qd is set to a class AB in which a slight bias current flows in the absence of signals.
In the conventional microwave amplifier in this manner, the input matching circuit 11 and the output matching circuit 12 effect the impedance matching between the input/output impedance of the transistor Qd and the external impedance.
However, the input/output impedance of the transistor may vary depending on the increase or decrease of the input signal power. For this reason, it will be difficult for the input matching circuit 11 and the output matching circuit 12 designed on the assumption that the transistor Qd has an unvaried input/output impedance to meet the matching conditions with the external impedance irrespective of the increase or decrease of the input signal power.
Accordingly as the input signal power increases, the base current increases and, depending on the diode characteristics across the base and emitter of the transistor Qd, the input impedance of the transistor Qd becomes smaller. As a result, the increased bias current allows the operation of the transistor Qd to shift from the class B operation to a class A operation. This results in an increased power consumption of the transistor Qd, which may place obstacles in power savings of battery-driven equipment such as cellular phones.
It is therefore an object of the present invention to provide a microwave amplifier capable of effecting a satisfactory impedance matching at all times irrespective of variations of the input signal power, to thereby ensure an effective feed of the input signal power to the amplifying element and suppress output signal distortions.
Another object of the present invention is to provide a microwave amplifier capable of restraining the input impedance from lowering even when the input signal power has increased to thereby impose a limitation on the increase in the power consumption of the amplifying element.
In order to achieve the above objects, an aspect of the present invention is characterized in that a resistor is inserted in series with an inductor feeding a bias current and that reactance of an input matching circuit or of an output matching circuit is varied depending on variations of input signal power. According to the present invention, the input impedance of the amplifying element is restrained from lowering even when the input signal power has increased so that a constantly satisfactory impedance matching is achieved irrespective of the input signal power variations. Furthermore, restriction on increase of the bias current enables a limitation to be imposed on increase of the power consumption of the microwave amplifier.
In order to attain the above objects, another aspect of the present invention is characterized by a microwave amplifier comprising a transistor arranged to amplify a microwave signal fed from a signal source to a base of the transistor; and a series circuit including an inductor and a resistor for feeding a bias current to the base of the transistor.
According to the present invention, due to the insertion of the resistor into a path for feeding a bias current to the base of the transistor, it is possible to restrict the increase of the bias current even when the input signal power has increased, whereupon a limitation is imposed on the lowering of the input impedance of the transistor as a result of the increased input signal power so that the impedance matching can be kept irrespective of the increase of the input signal power.
In order to attain the above objects, a further aspect of the present invention is characterized by a microwave amplifier comprising a first transistor arranged to amplify a microwave signal fed from a signal source to a base of the first transistor; and an input matching circuit in the form of an LC circuit interposed between the signal source and the base of the first transistor, the input matching circuit effecting an impedance matching between impedance of the signal source and input impedance of the first transistor, the input matching circuit including a first inductor and a first capacitance, either the first inductor or the first capacitance varying depending on power of the microwave signal so as to vary a reactance component of the input matching circuit.
According to the present invention, the reactance of the input matching circuit is varied depending on the variations of the input signal power whereby it is possible to effect an impedance matching between the signal source impedance and the first transistor input impedance irrespective of the input signal power variations.
Preferably, the first inductor includes a second transistor having a collector connected to a power source; a resistor disposed to feed a bias current to a base of the second transistor; a second capacitance connected across the base and an emitter of the second transistor; and a second inductor connected across the emitter of the second transistor and the ground; the microwave signal being fed to the emitter of the second transistor and fed via the first capacitance to the base of the first transistor.
According to the present invention, the transconductance of the second transistor increases accordingly as the input signal power increases, so that the first inductor lowers with increase of the input signal power. Thus, the first inductor becomes smaller accordingly as the input signal power increases whereby the impedance matching of the input matching circuit can be kept even when the input signal power has increased.
Preferably, the first capacitance includes a third transistor having a base and a collector that are connected to each other, with a bias current flowing between the base and an emitter of the third transistor, and the microwave signal is input to the emitter of the third transistor, the microwave signal is output from the base and collector of the third transistor to the base of the first transistor.
Preferably, the first capacitance includes a fourth transistor having an emitter and a collector that are connected to each other, with a bias current flowing between a base and the emitter and between the base and the collector of the fourth transistor, and the microwave signal is input to the emitter and the collector of the fourth transistor, the microwave signal is output from the base of the fourth transistor to the base of the first transistor.
According to the present invention, an increase of the input signal power results in an increased capacitance component as a result of thinning of the depletion layer in the forward bias of the first capacitance. Thus, in the event that the input signal power has increased with the lowered input impedance of the first transistor, it is possible to increase the value of the first capacitance and thereby to keep the impedance matching between the signal source internal impedance and the first transistor input impedance.